Wrist prosthesis

ABSTRACT

A wrist prosthesis has a radial part and a metacarpal part (12). A joint connection (15) connects the radial and the metacarpal parts including shafts for placement in the metacarpal bone and the carpal regions. The joint connection (15) is biaxial and permits both dorsal/palmar flexion and radial/ulnar deflection. The radial/ulnar flexion has a swivel radius (R 1 ). The dorsal/palmar flexion has a swivel radius (R 2 ). The radius (R 1 ) is greater than the radius (R 2 ). The contacting joint surfaces are congruent in all relative positions. A separate collar is provided for slipping onto the shaft of the radial part. The collar has a conical taper, a roughened surface and longitudinal ribs.

BACKGROUND OF THE INVENTION

The invention relates to a wrist prosthesis with a radial part having ashaft that can be received in the distal region of the radial medullarycanal and fixed therein, a metacarpal part having two shafts that extenddistally, one of which can be received and fixed within the proximalregion of the third metacarpal bone, whereas the other can be anchoredin the carpal region, and a joint connection between the radial part andthe metacarpal part.

A wrist prosthesis of this kind is generally known. Of the constructionsmost commonly used in practice, one is described in U.S. Pat. No.4,784,661 and others in U.S. Pat. No. 4,106,128, U.S. Pat No. 4,063,314,and U.S. Pat. No. 4,180,871. The three last publications are eachconcerned with wrist prostheses having a ball-joint connection betweenthe radial and the metacarpal part. Accordingly, these wrist prosthesesallow both dorsal-palmar flexion and radial-ulnar deflection plus axialrotation. The disadvantage of this joint connection is its deficientstability, denoted by the term "unconstrained" joint. The ball-jointconnection in itself would be ideal, as it most closely resembles thenatural wrist joint. However, if during implantation the mechanicalmidpoint of the ball-joint connection is not found in conjunction withthe tendons present in the wrist, a one-sided load is imposed on thejoint connection, with the consequence that it becomes incorrectlypositioned and is hence subject to abrasion. This effect is particularlyevident when polyethylene is used as bearing material. For this reasonthe useful life of such a wrist prosthesis is very limited.

The wrist prosthesis according to U.S. Pat. No. 4,784,661 has a jointconnection between the radial and metacarpal parts that comprises anellipsoidal joint element at the metacarpal part and a complementarybearing surface at the radial part. This joint prosthesis has come to bepreferred over the prosthesis with ball-joint connection. However, aproblematic feature of the construction according to U.S. Pat. No.4,784,661 is that axial rotation is not ruled out, and during suchrotation the ellipsoidal bearing element is lifted away from thecomplementary bearing surface. The same applies to angular movementsoutside the axes for flexion-extension and radial/ulnar deflection. Theresult is that the bearing surfaces wear out prematurely. Thepossibility of luxation is also not excluded.

SUMMARY OF THE PRESENT INVENTION

It is the object of the present invention to create a wrist prosthesisof the kind described at the outset that is characterized by a simpleand abrasion-resistant joint connection between the radial and themetacarpal part, whle corresponding closely to the mobility of theanatomical wrist joint.

The wrist prosthesis in accordance with the invention thus permits noaxial rotation between the radial and metacarpal parts. Axial rotationinstead occurs in the forearm. It has been found that the wrist itselfdoes not need to be capable of axial rotation. On the basis of thisfinding, the joint connection between radial and metacarpal part can bevery simply constructed, and in particular is substantially simpler thanin the case of the joint according to U.S. Pat. No. 4,307,473, whichlikewise incorporates a ball-joint connection. Furthermore, the choiceof swivel radii in accordance with the invention achieves a closeapproximation to the anatomical wrist. In this regard it should be keptin mind that the forces transmitted into the connective-tissuesupporting apparatus per angular degree are considerably less for"dorsal/palmar" than for "radial/ulnar". Correspondingly, in the wristprosthesis in accordance with the invention the radial/ulnar swivelradius is distinctly larger than the dorsal/palmar swivel radius.Preferably the radial/ulnar swivel radius is about 15 to 30 mm, inparticular 15 to 20 mm, whereas the dorsal/palmar swivel radius is onlyabout 2 to 5 mm, in particular 2 to 3 mm. Owing to the swivel radiichosen in accordance with the invention, the joint connection has asmaller tendency toward destabilization.

A self-centering function is achieved also provided by a rounded jointplate and a complementary bearing recess which is smaller at the edgesthan the central region. As the radial/ulnar angle increases, acorrespondingly increasing restoring force is exerted on the jointconnection. The result is the self-centering of the joint connection inthe radial/ulnar plane.

An important feature of a preferred joint connection is is the swivelradius formed by a rounded joint plate disposed at the end of ametacarpal part and a complementary recess in the radial part.Accordingly, the joint connection in accordance with this feativefunctions without striking a stopping element during both radial/ulnarand dorsal/palmar deflection. The two angular ranges are correspondinglylarge.

A specific construction is also distinguished by the fact that the jointsurfaces in contact with one another are always congruent in allpositions relative to one another. This is not the case with the nowpreferentially implanted wrist prosthesis according to U.S. Pat. No. 4,784,661, as is evident in the above comments regarding that prosthesis.That is, during axial rotation and during angular movements outside theplanes for flexion-extension and radial/ulnar deflection, theellipsoidal bearing surfaces lose contact with one another. Because thebearing or joint surfaces are no longer congruent, wear and tear isincreased, in some cases to the extent of loosening the prosthesis.

In another feature of a preferred construction, a collar preferrably aset of collars of different sizes is provided, which correspond to thevarious joint sizes or bone dimensions.

For cement-free implantation, the wrist prosthesis is preferably made oftitanium, and includes a bearing recess for the joint plate, which isshaped like a segment of a circle, is preferably part of a polyethylene,or ceramic insert. An insert made of bearing metal is also conceivable.

The rounded joint plate can also be made of ceramic or steel instead oftitanium. In principle, it is also conceivable to manufacture the wholewrist prosthesis of ceramic or steel. This would have the advantage thatthe wrist prosthesis would be suitable for both cement-free implantationand implantation with cement.

The surface of the collar, where provided is preferably roughened. Thecollar can in addition be provided with longitudinal ribs on thecircumference to create spongiosa spaces between the cortical substanceand the collar where provided and enable the bone tissue to grow intocontact with the implant surface (bony ingrowth).

An embodiment of a wrist prosthesis constructed in accordance with theinvention is described with reference to the attached drawing, wherein

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic longitudinal section through a wrist prosthesis inaccordance with the invention;

FIG. 2 is a plan view of the metacarpal part of the wrist prosthesis ofFIG. 1;

FIG. 3 is a plan view of the radial part of the wrist prosthesis of FIG.1;

FIG. 4 is a plan view of a collar to be set into the distal region ofthe radial medullary canal,

FIG. 5 is a bottom view of the collar of FIG. 4;

FIG. 6 is a schematic longitudinal section of the metacarpal joint partillustrates an approximately U-shaped slide-bearing surface; and

FIG. 7 is a perspective plan view of an alternate embodiment of acollar.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

In FIG. 1, a preferred embodiment of a wrist prosthesis is shown inschematic longitudinal section, with a radial part having a shaft 11that can be received by and fixed within the distal region of the radialmedullary c anal (not shown here). This shaft 11 is rod-like in shape,with an elongated oval cross section. This cross section corresponds toa similarly shaped axial bore through a collar like that shown in FIGS.4, 5 or 7, which can be slid over the shaft 11 and placed in the distalregion of the radial medullary canal. Said collar is identified by thereference numeral 19 in FIGS. 4, 5 by 20 in FIG. 7.

The wrist prosthesis includes a metacarpal part 12 with two distallyextending shafts 13 and 14 of different lengths, the longer of which isto be received by and fixed within the proximal region of the thirdmetacarpal bone. The shorter shaft is anchored in the carpal region,(preferably in the scaphoid bone). In this regard the design is knownper se. Between the metacarpal part 12 and the radial part 10 isdisposed a joint connection 15. This is biaxial in such a way as topermit, firstly, dorsal/palmar flexion and, secondly, radial/ulnardeflection corresponding to the double-headed arrow 21. The constructionis designed such that the radial/ulnar swivel radius R₁ is larger thanthe dorsal/palmar swivel radius R₂. Preferably, the radial/ulnar swivelradius R₁ is greater than the dorsal/palmar swivel radius R₂ by a factorof about 5 to 15, in particular about 10. The radial/ulnar swivel radiusR₁ is defined by the interplay between a rounded joint plate 16, shapedlike a segment of a circle, at the proximal end of the metacarpal part12 and a complementary bearing recess 17 at the distal end of the radialpart 10. In correspondence with the shape of the joint plate 16, thecomplementary bearing recess 17 is a groove with approximately circularfloor with a common axis 17.

The two metacarpal shafts 13 and 14 are coupled to the rounded jointplate 16 by way of a flat connection element 22, mounted on a joint axle18 extending from "radial" to "ulnar" which defines the dorsal/palmarswivel radius R₂. That is, the dorsal/palmar swivel radius R₂ equalshalf the diameter of the joint axle 18 as shown by axis 18.

The said bearing recess 17 to receive the rounded joint plate 16 in theembodiment shown here is part of a polyethylene insert that can befitted into a potlike receptacle made of metal, in particular titaniumor titanium alloy. The above-mentioned shaft 11 is attached to the floorof the receptacle 23.

As shown in FIG. 6, the radius R_(S) of the rounded joint plate 16 andthe corresponding radius of the complementary bearing recess 17 can besmaller at the edges than in the central region, in particular canbecome continuously smaller toward the two edges, so as to form anapproximately U-shaped slide-face contour of the joint plate 16 andbearing recess 17. By this means the above-mentioned self-centering ofthe joint connection in the radial/ulnar plane is achieved.

In addition, both FIG. 1 and FIG. 6 make clear that the joint connection15 is offset in the ulnar direction from the axis of the radial shaft11.

Onto the shaft 11 disposed on the radial part 10, as shown in FIG. 3, acollar 19 can be slipped so that when the radial part is in theimplanted state, the collar 19 is seated in the distal region of theradial medullary canal. The collar 19 tapers conically from distal toproximal. As shown in FIG. 5, it is shaped like an elongated oval incross section. The same applies to the axial bore 24 passing through thecollar, which corresponds to the elongated oval cross section of theshaft 11. Hence the collar cannot rotate when it is in place on theshaft 11, where it makes contact with the floor of the receptacle 23.

As shown in FIG. 7, the collar 20 can be constructed with longitudinalribs over its circumference. The longitudinal ribs are identified inFIG. 7 by the reference numeral 25. These create spongiosa spacesbetween cortical substance and collar 20, which ensure free passage ofmaterials to the distal part of the bone and permit the bone tissue togrow toward and into the spaces as far as the implant surface.

The length of the longer shaft 13 on the metacarpal part 12 is about 35to 45, in particular about 40 mm. The coupling of the shafts 13 and 14to the joint plate 16 allows dorsal inclintion of about 7 to 10, inparticular about 8°. The two shafts 13 and 14 extend approximatelyparallel to one another. The smaller shaft 14 has a length of about 12to 20, in particular about 15 mm.

The length of the receptacle 23 is about 30 to 35 mm. Its width is about12 to 20 mm, in particular about 15 to 16 mm. The cross section of theshaft 11 measures about 3×4 mm. The shaft 11 is preferably offset fromthe geometrical central axis not only toward the radius but alsodorsally.

The cross section of the joint plate 16 and, correspondingly, of thebearing recess 17 is preferably rectangular, though the interior edgesmay be more or less strongly rounded.

The collar 19 or 20 extends axially for about 17 to 25 mm, in particularabout 20 mm. The length of the shaft 11 is about 40 to 50, in particularabout 45 mm. The height of the receptacle 23 is about 10 to 15, inparticular about 12 mm.

The minimal wall thickness of the polyethylene insert or inlay withinthe receptacle 23 is about 2 to 5 mm.

The above dimensions are average values, from which departures may bemade in individual cases. Ultimately the dimensions depend on the sizeof the wrist in which the prosthesis is to be implanted.

All the characteristics disclosed in the application documents areclaimed as essential to the invention, to the extent that they are newto the state of the art singly or in combination.

List of reference numerals

10 radial part

11 shaft

12 metacarpal part

13 shaft

14 shaft

15 joint connection

16 joint plate shaped like a segment of a circle

17 bearing recess

18 joint axle

19 collar

20 collar

21 double-headed arrow

22 connection element

23 receptacle

24 bore

25 longitudinal rib

26 polyethylene insert

I claim:
 1. A wrist prosthesis with a radial part (10) bearing a shaft(1) that is configured to be received in the distal region of the radialmedullary canal fixed therein, a metacarpal part (12) including twoshafts (13, 14) that extend distally, and of which a first shaft isconfigured to be fixed in the proximal region of the third metacarpalbone and the second shaft is configured to be anchored in the carpalregion, comprising a joint connection (15) disposed between the radialpart and the metacarpal part configured to produce only biaxial movementin said wrist prosthesis, said biaxial movement of said joint connectionhaving a dorsal/palmer flexion with a swivel axis (17') and aradial/ulnar deflection with a swivel axis (18'), the radial/ulnarswivel axis having a radius (R₁) and said dorsal/palmer swivel axishaving a radius (R₂) and wherein said radial/ulnar deflection includes arounded joint plate (16) generally in the shape of a segment of a circleand disposed at the proximal end of the metacarpal part (12) and saidradial part (10) having a complementary bearing recess (17) at a distalend, and said radial/ulnar swivel axis (17') is greater than saiddorsal/palmer swivel radius (18').
 2. The wrist prosthesis of claim 1wherein said radial/ulnar swivel radius (R₁) is greater than thedorsal/palmar swivel radius (R₂) by a factor of 5-15.
 3. The wristprosthesis of claim 2 wherein said factor is substantially
 10. 4. Thewrist prosthesis of claim 1 having said rounded joint plate (16) whereinsaid two metacarpal shafts (13,14) are coupled to said rounded jointplate (16) with a joint axle (18) extending from said radial part tosaid ulnar part and forms said axis 18' and dorsal/palmar swivel radius(R₂).
 5. The wrist prosthesis of claim 1 wherein the bearing recess (17)for the rounded joint plate (16) includes a bearing insert of a materialselected from the group consisting of polyethylene, ceramic andbearing-metal.
 6. The wrist prosthesis of claim 1 wherein said roundedjoint plate (16) has a radius (R_(s)), and said complementary bearingrecess (17) having a radius smaller than said radius (R_(s)) at theedges and extending toward a radial and ulnar edge than the centralregion of the recess.
 7. The wrist prosthesis of claim 1 wherein saidrecess (17) is continuously smaller from the central region to at leastone of the radial and/or ulnar edge.
 8. The wrist prosthesis of claim 1wherein said joint connection (15) is offset from the axis of said shaft(11) of said radial part in the direction of the ulnar deflection. 9.The wrist prosthesis of claim 1 includes at least one collar (19; 20) onsaid shaft (11) of the radial part (10).
 10. The wrist prosthesis ofclaim 9 wherein said collar (19; 20) has a conical taper from a distalportion to a proximal portion, said collar has a circular cross-section.11. The wrist prosthesis of claim 10 wherein said conical taper has anelongated oval cross section.
 12. The wrist prosthesis of claim 10wherein said collar (20) includes longitudinal ribs (25) over itscircumference.